Bearing unit and rotary drive apparatus including the bearing unit

ABSTRACT

A reliable bearing unit, in which lubricant does not leak and the problem of shaft lifting at the time of the rotation of a rotor due to the imbalance of a pair of dynamic pressure generating grooves can be surely and inexpensively solved, and a rotary drive apparatus including the bearing unit are provided.  
     Accordingly, a shaft  100  which has an exposed end  160,  an inner end  162  of a small external diameter provided opposite to the exposed end  160,  and a stepped middle portion  170  formed between the exposed end  160  and the inner end  162;  a thrust bearing  130  which supports the inner end  162  of the shaft  100  in a rotatable manner in the thrust direction; and lubricant  150  in a retention member  120  which is filled between the shaft  100,  a radial bearing  110  and the thrust bearing  130  are provided; and the length m of the inner end  162  of the shaft  100  in the direction of the shaft is shorter than the length n in the direction of the shaft of the part between the outer surface of the retention member  120  and the stepped middle portion  170  of the shaft  100.

TECHNICAL FIELD

The present invention relates to a bearing unit which supports a shaftin a rotatable manner, and to a rotary drive apparatus including thebearing unit.

BACKGROUND ART

A bearing unit supports a shaft in a rotatable manner, and the bearingunit is provided in a motor of a disk apparatus, for example.

A bearing unit having such structure is the one in which an I-shaped(otherwise called straight-type) shaft is supported in a rotatablemanner using lubricant. (Refer to, for example, patent reference 1 andpatent reference 2.)

[Patent reference 1]

Japanese Published Patent Application No. 2002-27703 (page 1, FIGS. 1and 2)

[Patent reference 2]

Japanese Published Patent Application No. H8-335366 (page 1, FIGS. 1 and2)

DISCLOSURE OF THE INVENTION

A bearing unit installed in a motor of patent reference 2 ischaracterized in that the width B1 of a dynamic pressure generatinggroove on the side of the motor where a rotor portion is installed islarger than the width B2 of a dynamic pressure generating groove on theside where a rotor portion is not installed, as shown in FIG. 2 inpatent reference 2.

The purpose of enlarging the width B1 of the dynamic pressure generatinggroove on the rotor portion side is to improve rigidity when the rotorportion rotates; and also other effectiveness can be obtained as well.

The following is the other effectiveness. A shaft (fixed shaft whichdoes not rotate in this case) and dynamic pressure bearing rotaterelatively, and when the dynamic pressure generating grooves generatedynamic pressure, the shaft moves from the high static pressure side tothe low static pressure side. In other words, since the shaft moves fromthe low dynamic pressure side to the high dynamic pressure side, in thecase of a motor the shaft is moved in the direction from the narrowdynamic pressure generating groove not capable of generating muchdynamic pressure to the broad dynamic pressure generating groove capableof generating a lot of dynamic pressure. Specifically, the shaft ispressed to a thrust bearing due to the relative rotation with thedynamic pressure bearing, so that rigidity is increased.

As regards a motor of Patent reference 2, in order to obtain rigiditywhen the rotor portion rotates, also the width B1 of a dynamic pressuregenerating groove on the rotor portion side is larger than the width B2of a dynamic pressure generating groove on the non rotor portion side inthe bearing unit shown in FIG. 1(b).

However, in the case of that motor, the shaft moves from the low dynamicpressure side to the high dynamic pressure side in the manner describedabove when the shaft rotates, so that the shaft starts to liftconcurrently with the rotation.

The power of the dynamic pressure is great enough to lift the rotorportion a great deal, so that regarding a motor for an HDD (hard diskdrive) for example, the machine accuracy of the disk and recording headinstalled in the motor can not be maintained. Thus, problems of notcapable of performing normal recording and reproduction are caused.Also, regarding a fan motor or the like, there is a risk of a fan beingcontact with the surrounding parts.

The bearing units shown in FIGS. 1 and 2 in patent reference 2 are wellin relatively changing the widths of the dynamic pressure generatinggrooves to improve the rigidity of the motors; however, not in the caseof a fixed shaft, in the case of the shaft-rotating type there has beendisadvantage in which a rotor portion lifts along with a shaft.Specifically, the dynamic pressure must be continuously low on the sidewhere a shaft is exposed from a dynamic pressure bearing.

A dynamic pressure bearing apparatus of patent reference 1 ischaracterized in that a dynamic pressure generating groove on theshaft-exposing side is herringbone-shaped, and also the depth of half ofthe groove on the half of the shaft-exposing side is larger than thedepth of a groove on the non-exposing side. There is mentioned anadvantage in which, when change in a groove is provided regarding onedynamic pressure generating groove, lubricant flows toward the inside ofa bearing unit to prevent from leaking. As described above, a shaftmoves from the low dynamic pressure side to the high dynamic pressureside, in other words, the shaft is moved toward the inside direction, sothat the shaft is pulled effectively also in patent reference 1.

However, since the depth of the dynamic pressure generating grooveprocessed must be controlled with high accuracy by a rolling ortransferring method, etching, electric discharging, and the like in thecase of patent reference 1, it is difficult in practice to do so, and ifit is made, there will be a disadvantage of increasing cost.

Accordingly, in light of the above problems, the present invention aimsto provide a bearing unit in which lubricant does not leak to secure thereliability and the problem of shaft lifting at the time of the rotationof a rotor caused due to the imbalance of a pair of dynamic pressuregenerating grooves can be solved reliably and inexpensively; and toprovide a rotary drive apparatus including the bearing unit.

The present invention provides a bearing unit that supports a shaft in arotatable manner, including: a shaft which has an exposed end, an innerend having a small external diameter provided opposite to the exposedend, and a stepped middle portion having a small external diameterformed at a position between the exposed end and the inner end; aretention member which exposes the exposed end of the shaft to theoutside through a gap and has a seamless structure; a bearing disposedinside the retention member, which has a first dynamic pressuregenerating groove on the exposed end side and a second dynamic pressuregenerating groove on the inner end side formed on an innercircumferential surface facing the shaft and which supports the shaft ina rotatable manner in the radial direction; a thrust bearing formedinside the retention member, which supports the inner end of the shaftin a rotatable manner in the thrust direction; and lubricant in theretention member, which is filled between the shaft, the radial bearingand the thrust bearing; wherein, the length m of the inner end of theshaft in the direction of the shaft is shorter than the length n in thedirection of the shaft of the part between the outer surface of theretention member and the stepped middle portion of the shaft.

According to the present invention, a shaft has an exposed end, an innerend and a stepped middle portion. The inner end is a part having a smallexternal diameter provided opposite to the exposed end. The steppedmiddle portion is a part having a small external diameter positionedbetween the exposed end and the inner end.

A retention member exposes the exposed end of the shaft to the outsidethrough a gap and has a seamless structure.

A bearing has a first dynamic pressure bearing and a second dynamicpressure bearing, and supports the shaft in a rotatable manner in theradial direction.

A thrust bearing is formed inside the retention member. The thrustbearing supports the inner end of the shaft in a rotatable manner in thethrust direction.

Lubricant is provided in the retention member and is filled between theshaft, the radial bearing and the thrust bearing.

The length m of the inner end of the shaft in the direction of the shaftis shorter than the length n in the direction of the shaft from theouter surface of the retention member to a part including the steppedmiddle portion of the shaft.

Therefore, the dynamic pressure of the inner end of the shaft, which isthe non shaft-exposing side, can be set higher than the dynamic pressureof the exposed end, which is the shaft-exposing side. Accordingly, theshaft can be formed with ease, and the shaft can be pulled toward theinside of the retention member, so that the problem of the shaft liftingcan be solved reliably and inexpensively.

Furthermore, since the lubricant is always pulled toward the inside ofthe retention member as well and the retention member has a seamlessstructure, an excellent bearing unit without causing a leakage oflubricant can be provided reliably and inexpensively.

Further, according to the present invention, in the above-describedbearing unit, the inner end is a tapered portion or a stepped portionhaving a small external diameter.

In the present invention, an inner end is a diminishing tapered portionor a stepped portion having a small external diameter.

Further, according to the present invention, in the above-describedbearing unit, the external diameter D of the inner end is larger thanthe external diameter d of the stepped middle portion.

According to the present invention, the external diameter D of an innerend is larger than the external diameter d of a stepped middle portion.

Accordingly, the dynamic pressure on the non shaft-exposing side is madehigher than the dynamic pressure on the shaft-exposing side, so that theproblems of a shaft lifting and of lubricant leakage can be solved.

Further, according to the present invention, in the above-describedbearing unit, the stepped middle portion is a stepped portion formed sothat the peripheral portion of the shaft on the exposed end side facingthe first dynamic pressure generating groove becomes smaller.

Further, according to the present invention, in the above-describedbearing unit, the first and second dynamic pressure generating groovesare herringbone grooves, and the inflow angle α of the first dynamicpressure generating groove is larger than the inflow angle β of thesecond dynamic pressure generating groove.

According to the present invention, since the inflow angle α of thefirst dynamic pressure generating groove is larger than the inflow angleβ of the second dynamic pressure generating groove, the dynamic pressureof the dynamic pressure generating groove on the non shaft-exposing sidecan be higher than the dynamic pressure on the shaft-exposing side.

Further, the present invention provides a rotary drive apparatus havinga bearing unit that supports a shaft in a rotatable manner, including: ashaft which has an exposed end, an inner end of a small externaldiameter provided opposite to the exposed end, and a stepped middleportion formed at a position between the exposed end and the inner end;a retention member which exposes the exposed end of the shaft to theoutside through a gap and has a seamless structure; a bearing disposedinside the retention member, which has a first dynamic pressuregenerating groove on the exposed end side and second dynamic pressuregenerating groove on the inner end side formed on an innercircumferential surface facing the shaft and which supports the shaft ina rotatable manner in the radial direction; a thrust bearing formedinside the retention member, which supports the inner end of the shaftin a rotatable manner in the thrust direction; and lubricant in theretention member, which is filled between the shaft, the radial bearingand the thrust bearing; wherein the length m of the inner end of theshaft in the direction of the shaft is shorter than the length n in thedirection of the shaft of the part between the outer surface of theretention member and the stepped middle portion of the shaft.

According to the present invention, a shaft has an exposed end, an innerend and a stepped middle portion. The inner end is a part having a smallexternal diameter provided opposite to the exposed end. The steppedmiddle portion is a part having a small external diameter positionedbetween the exposed end and the inner end.

A retention member exposes the exposed end of the shaft to the outsidethrough a gap and has a seamless structure.

A bearing has a first dynamic pressure bearing and second dynamicpressure bearing, and supports the shaft in a rotatable manner in theradial direction.

A thrust bearing is formed inside the retention member. The thrustbearing supports the inner end of the shaft in a rotatable manner in thethrust direction.

Lubricant in the retention member is filled between the shaft, theradial bearing and the thrust bearing.

The length m of the inner end of the shaft in the shaft direction isshorter than the length n in the direction of the shaft from the outersurface of the retention member to a part including the stepped middleportion of the shaft.

Therefore, the dynamic pressure of the inner end of the shaft, which ison the non shaft-exposing side, can be set higher than the dynamicpressure of the exposed end, which is on the shaft-exposing side.Accordingly, the shaft can be made with ease and pulled toward theinside of the retention member, so that the problem of the shaft liftingcan be solved reliably and inexpensively.

Furthermore, since the lubricant is always pulled toward the inside ofthe retention member as well and the retention member has a seamlessstructure, a rotary drive apparatus having an excellent bearing unitwhich does not cause lubricant leakage can be provided reliably andinexpensively.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing an example of electronic equipmentincluding a bearing unit of the present invention;

FIG. 2 is a cross-sectional view of a fan motor used in FIG. 1;

FIG. 3 is a perspective view of the fan motor shown in FIG. 2;

FIG. 4 is a cross-sectional view showing the fan motor in detail;

FIG. 5 is a cross-sectional view showing the bearing unit in an enlargedscale;

FIGS. 6A and 6B are views showing examples of the shape of a first andsecond dynamic pressure generating grooves of the bearing unit;

FIG. 7 is a view showing the part A in FIG. 5 in an enlarged scale;

FIG. 8 is a diagram showing an example of the dynamic pressure at theinflow angle in the dynamic pressure generating groove; and

FIG. 9A is a view showing the first dynamic pressure generating grooveand FIG. 9B is a view showing an example of the dynamic pressure of thesecond dynamic pressure generating groove.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail based on the accompanying drawings.

It should be noted that since the following embodiments are favorablespecific examples, various limitations that are technically preferableare given thereto; however, the scope of the present invention is notlimited to those embodiments unless there is a particular description oflimiting the present invention in the following explanation.

FIG. 1 shows a portable computer 1 as an example of electronic equipmentto which a motor including a bearing unit of the present invention isapplied.

The computer 1 has a display portion 2 and a main body 3, and thedisplay portion 2 is joined to the main body 3 in a rotatable manner bymeans of a joining portion 4. The main body 3 has a keyboard 5 and achassis 12. In the chassis 12 is provided a heat sink device 10.

FIG. 2 shows an example of a cross-sectional structure taken by an E-Eline of the chassis 12 in FIG. 1. FIG. 3 is a perspective view showingan example of the construction of the heat sink device 10 provided inthe chassis 12 shown in FIG. 2.

In FIG. 2, the heat sink device 10 is accommodated in the chassis 12.The heat sink device 10 has a structure shown in FIG. 3. The heat sinkdevice 10, also called a cooling device, has a metal base 20; a motor30; a fan 34 which is a target object for rotation; a fan case 36; and aheat sink 38.

One surface (corresponding to the lower surface) 21 of the base 20 has amounting surface 50, a mounting surface 52 and a mounting surface 54.The mounting surfaces 50, 52 and 54 are formed to be, for example,approximately L-shaped, and a heat emitting element 40 is fixed to onesurface 21 of the mounting surface 50 with a heat transfer seal 44. Theheat emitting element 40 is, for example, a CPU (central processingunit), which emits heat when operated by applying current.

The fan case 36 and the motor 30 are fixed to the mounting surface 52.Inside the fan case 36 are accommodated the fan 34 and the motor 30. Thefan case 36 has a circular hole 48. The circular hole 48 is formed at aposition facing a hole 60 in the lower surface of the chassis 12 asshown in FIG. 2. The fan case 36 has a hole 37 on the side of the heatsink 38 that is a target object for cooling, to which cooling wind isprovided.

The heat sink 38 is fixed to the mounting surface 54. The heat sink 38is, for example, a corrugated or fin-shaped heat sink, and is made of ametal such as aluminum, for example that is superior in heatdissipation. The base 20 and the fan case 36 can be made of a metal thatis superior in heat dissipation such as aluminum or iron.

A hole 70 is provided in necessary positions in the base 20, and thebase 20 is fixed with a screw on the inner surface side of the chassis12 through a boss 72 in FIG. 2, using these mounting holes 70.

The heat sink 38 shown in FIGS. 2 and 3 is provided at a positioncorresponding to a hole 76 in the side surface of the chassis 12.Accordingly, when the motor 30 is operated to make the fan 34 rotate inthe R direction continuously, air inside the chassis 12 is expelled fromthe hole 76 in the side surface to the outside, via the arrows D1, D2and D3 after taking in through the holes 60 and 48.

At that time, since heat emitted from the heat emitting element 40reaches the mounting surface 54 via the mounting surfaces 50 and 52 ofthe base 20, the heat of the heat emitting element 40 reaches the heatsink 38. A flow of air generated by the rotation of the fan 34 runsalong the arrows D1, D2 and D3, and the heat conveyed to the heat sink38 can be emitted outside through the hole 76 in the side surface of thechassis.

FIG. 4 shows an example of a cross-sectional structure of the motor 30of FIG. 3. The motor 30 has a rotor 80 and a stator 84.

The motor 30 and the fan 34 are accommodated in the fan case 36, and thestator 84 is integrally provided on the upper surface portion 36A sideof the fan case 36. The stator 84 has a stator yoke 88, a bearing unit90, a coil 164 and a core 160.

The stator yoke 88 may be formed integrally with the upper surfaceportion 36A of the fan case 36 or may be formed separately, and is madeof iron or stainless steel, for example. A housing 120 of the bearingunit 90 is fixed inside a holder 92 of the stator yoke 88 by pressfitting or adhesive bonding or by both. The holder 92 is a cylindricalpart.

Schematically, the bearing unit 90 shown in FIG. 4 includes a shaft 100,a radial bearing 110, a thrust bearing 130, the retention member (alsocalled housing) 120 and lubricant 150.

FIG. 5 shows the bearing unit 90 shown in FIG. 4 further in detail. Thestructure of the bearing unit 90 will be explained in more detail,referring to FIG. 5.

The shaft 100 is a so-called I-shaped (otherwise called straight-type)shaft. The shaft 100 is made of stainless steel, for example.

The shaft 100 has the exposed end 160, a shaft peripheral portion 161,an inner end 162, a stepped middle portion 170 and a tapered portion100A.

The external diameter of the exposed end 160 can be the same as that ofthe shaft peripheral portion 161.

The tapered portion 100A is a taper-shaped part positioned between theexposed end 160 and the shaft peripheral portion 161. The taperedportion 100A diminishes in diameter from the shaft peripheral portion161 toward the exposed end 160. The exposed end 160 is exposed to theoutside from a gap S of the retention member 120, and the taperedportion 100A is formed at a position corresponding to the gap S.

The inner end 162 of the shaft 100 is supported by the thrust bearing130 of the retention member 120 in a rotatable manner in the thrustdirection. The shape of the inner end 162 may be such a stepped shape asshown in FIG. 5, or may be a tapered shape, needless to say. In the caseof a tapered shape, the inner end 162 has a diminishing tapered shape.The diameter of the inner end 162 is shown by D, and the length of theinner end 162 in the direction of the shaft is shown by m.

As shown in FIG. 5, the stepped middle portion 170 is formed in themiddle of the shaft 100. The diameter of the stepped middle portion 170is shown by d. Preferably, the stepped middle portion 170 is formed of astepped portion 171, a peripheral portion 179 and a part of theabove-described tapered portion 100A.

The diameter D of the inner end 162 is set larger than the diameter d ofthe peripheral portion 179. Also, the length m of the inner end 162 inthe shaft direction is set shorter than the length n of a part from anend surface 121 of the retention member 120 to the stepped middleportion 170 included.

As described above, the external diameter D is set larger than theexternal diameter d (D>d). Further, preferably the length m of the innerend 162 in the shaft direction is set shorter than the length n of thestepped middle portion 170 in the shaft direction.

Next, the radial bearing 110 shown in FIG. 5 is to be explained.

The radial bearing 110 is a cylindrical member, and supports the shaftperipheral portion 161 of the shaft 100 in a rotatable manner in theradial direction. As an example, a first dynamic pressure generatinggroove 201 and a second dynamic pressure generating groove 202 areformed at a interval in the inner circumferential surface of the radialbearing 110. The first dynamic pressure generating groove 201 is formedin the vicinity of the stepped middle portion 170, preferably such thatthey overlap. The second dynamic pressure generating groove 202 isformed on the inner end 162 side. The first dynamic pressure generatinggroove 201 can be referred to as a dynamic pressure generating groove onthe shaft-exposing side. The second dynamic pressure generating groove202 can be referred to as a dynamic pressure generating groove on thenon shaft-exposing side.

The radial bearing 110 can be made of a metal such as brass or stainlesssteel, a sintered metal, or the like. In the case where a sintered metalor a metal is used, such a dynamic pressure generating groove as aherringbone groove can be formed through a technique such as rolling,transferring, electric discharge, etching processing, and so on.

FIG. 6A shows an example of the shape of the first dynamic pressuregenerating groove 201, and FIG. 6B shows an example of the shape of thesecond dynamic pressure generating groove 202. It is desirable that theinflow angle α for lubricant of the first dynamic pressure generatinggroove 201 be set larger than the inflow angle β for lubricant of thesecond dynamic pressure generating groove 202.

The retention member 120 shown in FIG. 5 is a member of a seamlessstructure having the gap S. The retention member 120 is not formed bycombining a plurality of members but formed by employing a polymericmaterial such as Teflon®, polyimido, polyamido, LCP (liquid crystallinepolymer) or PC (polycarbonate), or a sintered metal, using outsertmolding onto the radial bearing 110.

Although the retention member 120 is provided with the tiny gap S asdescribed above, there is a seamless structure around its periphery. Theretention member 120 accommodates the radial bearing 110 and the shaftperipheral portion 161 of the shaft 100. The lubricant 150 is filledbetween the shaft peripheral portion 161, the radial bearing 110 and theretention member 120.

Since the tiny gap S has a cross-section that is a tapered shape, apressure gradient is generated to form a surface tension seal forlubricant to be pulled inside the bearing unit.

Note that as shown in FIG. 6A, the width W of the first dynamic pressuregenerating groove 201 in the direction of the shaft is set larger thanthe width W1 of the second dynamic pressure generating groove 202 shownin FIG. 6B in the direction of the shaft. However, not limited thereto,the width W may be set smaller than the width W1.

Hereupon, advantages obtained by providing magnitude relation betweenthe above-described respective dimensions will be explained.

The dynamic pressure Pd generated when the shaft 100 relatively rotatesis proportional to the square of the flow velocity u of lubricant, to bePd∝u².

Since the flow velocity u is proportional to the relative velocity U ofthe shaft 100 and is inversely proportional to the amount h of the gapbetween the shaft 100 and the radial bearing 110, to be u∝U/h. Here,U=rω, r: shaft radius, ω: shaft rotation speed is obtained.

Specifically, since the dynamic pressure Pd is virtually proportional tothe square of the shaft radius r and is inversely proportional to thesquare of the amount c of the gap between the shaft and the bearing,Pd∝(r/c) is obtained.

As a result, the smaller the external diameter of the shaft is made, themore the generation of dynamic pressure can be reduced.

In the bearing unit 90 of FIG. 5 in the present invention, since thestepped portion of the inner end 162 on the non shaft-exposing side isshorter in shaft length (m<n) and also is larger in shaft diameter (D>d)than the stepped middle portion 170 on the shaft-exposing side as shownin FIG. 5, the dynamic pressure on the shaft-exposing side is alwayslower.

Moving from the low dynamic pressure side to the high dynamic pressureside, the shaft 100 is pulled toward the thrust bearing 130 inside theretention member 120 and therefore never lifts.

Furthermore, in order to prevent the shaft 100 in FIG. 5 from lifting,the stepped middle portion 170 is provided at the part of the shaft 100facing the first dynamic pressure generating groove 201 so that thedynamic pressure of the second dynamic pressure generating groove 202 onthe non shaft-exposing side becomes higher than the dynamic pressure ofthe first dynamic pressure generating groove 201 on the shaft-exposingside.

Further, by providing the stepped middle portion 170, not only thedynamic pressure of the first dynamic pressure generating groove 201 onthe shaft-exposing side becomes smaller than that of the second dynamicpressure generating groove 202 on the non shaft-exposing side, but alsothe first dynamic pressure generating groove 201 itself on theshaft-exposing side can make the shaft-exposing side to have lowerdynamic pressure, and the shaft 100 can be prevented from lifting evenmore reliably and inexpensively. In other words, the inner end 162 andthe stepped middle portion 170 can be made with ease on the shaft.

In prior art, it is attempted to reduce the dynamic pressure on theshaft-exposing side by changing the depth of a dynamic pressuregenerating groove, so that oil flow toward the inside, as describedabove. In the bearing unit of the present invention, however, since theexternal diameter of the shaft 100 is changed to obtain the change ofthe dynamic pressure, it is possible to produce the bearing unit by farmore simplified and inexpensive manner, and to obtain the sameeffectiveness reliably.

To briefly explain the above-described effectiveness, when the dynamicpressure on the shaft-exposing side and the dynamic pressure on the nonshaft-exposing side are compared, the pressure on the shaft-exposingside is low according to the shape of the shaft 100, and theshaft-exposing side thereof is low according to the dynamic pressuregenerating groove. Hence, the dynamic pressure of the shaft-exposingside is set low in every respect, so that the shaft can be reliablyprevented from lifting. To put it another way, the bearing unit 90 canreliably make lubricant flow into the bearing unit to be retained and isprovided inexpensively with much reliability.

Hereupon, the necessity of providing a stepped part as the inner end 162on the non shaft-exposing side shown in FIG. 5 will further beexplained.

Conventionally, in order to prevent lubricant from leaking, a pluralityof members are used to surround a bearing unit; however, since it hasnot been easy to seal joints thereof, it has been necessary to apply apacking agent such as epoxy resin or has required other means, so thatthe cost became high and unreliability were inevitable.

In the bearing unit 90 of the present invention, the surroundingretention member 120 is formed by making a polymeric material such asLCP undergo outsert molding and is made completely seamless except forthe surface tension seal portion in the gap S portion to be providedinexpensively and reliably.

However, as shown in FIG. 7, the temperature of the retention member 120returns to a normal temperature after outsert molding is performed onresin at a high temperature, for example, approximately between 100° C.to 250° C. At that time, an edge E of the retention member 120 slightlyprojects on the inner circumferential side of the radial bearing 110 dueto a difference between the contraction percentage of the radial bearing110 made of a sintered metal or the like and that of the retentionmember 120 made of a polymeric material. In order to avoid the edge Ecoming to contact with the shaft 100, a contact-prevention means such asthe stepped portion of the inner end 162 of the shaft 100 or a taperedshape is necessary.

Specifically, if the seamless retention member 120 made of a polymericmaterial is used for completely preventing lubricant from leaking, thecontact-prevention means such as the stepped portion of the inner end162 or the like is needed, and as a result, the dynamic pressuregenerated based on the shape of the shaft needs to be adjusted, so thata structure according to the present invention is required.

However, the purpose of the bearing unit 90 of the present invention isto prevent the shaft from lifting, so that the structure of theretention member may not be limited at all.

As shown in FIGS. 6A and 6B, in order to make the dynamic pressure onthe shaft-exposing side relatively lower than the dynamic pressure onthe non shaft-exposing side, the dynamic pressure generating grooves 201and 202 are made to be of a herringbone type, and further, theherringbone inflow angles α and β can be favorably selected.

FIG. 8 shows calculation results of the dynamic pressure when the inflowangle of the herringbone groove was 20°, 30° and 40° respectively. Thehorizontal axis shows the ratio of the amount c of the gap between theshaft 100 and the radial bearing 110 to the sum of the gap amount c andthe depth h of the herringbone groove, (h+c)/c. The vertical axis showsthe dynamic pressure generated.

The larger the inflow angle becomes, reaching 30° and 40° , compared tothe case of the inflow angle of 20° the lower the dynamic pressurebecomes, so that by making the inflow angle α of the first dynamicpressure generating groove 201 on the shaft-exposing side larger thanthe inflow angle β of the second dynamic pressure generating groove 202on the non shaft-exposing side, even more reliably the shaft 100 can beprevented from lifting.

Referring to FIGS. 9A and 9B, specific examples of the structural designare further described.

In FIG. 9A, the horizontal axis shows the ratio of the gap amount c tothe sum of the gap amount c and the depth h of the herringbone groove,(c+h)/c, and the vertical axis shows the dynamic pressure. The gapamount c and the groove depth h are shown in FIG. 9B.

One of the characteristic curves shows the dynamic pressure of the firstdynamic pressure generating groove 201 on the shaft-exposing side, andthe other shows the dynamic pressure of the second dynamic generatinggroove 202 on the non shaft-exposing side.

Here, even if the machine-made dimensions of the gap amount c and thegroove depth h are widely varied, the dynamic pressure on the nonshaft-exposing side must be higher than the dynamic pressure on theshaft-exposing side invariably.

For example, if the gap amount c=1 to 2 μm and the groove depth h=2 to 3μm is set, (c+h)/c becomes (2+2)/2=2 in minimum value and becomes(1+3)/1=4 in maximum value, so that within the range for use marked byoblique lines in FIG. 9A, the dynamic pressure of the second dynamicpressure generating groove 202 on the non-exposing side is always kepthigher than the dynamic pressure of the first dynamic pressuregenerating groove 201, and therefore the problem of the shaft liftingcaused by the dispersion resulting from machine accuracy cannot occur.

Therefore, the above-described various improvements should be made sothat the dynamic pressure on the non shaft-exposing side becomesinvariably higher than the dynamic pressure on the shaft-exposing side.

As described above, the bearing unit of the present invention has thefollowing advantages.

According to the bearing unit 90 of the present invention, the dynamicpressure on the non shaft-exposing side is set higher than the dynamicpressure on the shaft-exposing side. Specifically, the dynamic pressureof the second dynamic pressure generating groove 202 on the nonshaft-exposing side is set higher than that of the first dynamicpressure generating groove 201 on the shaft-exposing side, by providingthe stepped middle portion 170 on the shaft facing the first dynamicpressure generating groove 201 on the shaft-exposing side and by varyingthe inflow angles α and β.

As regards the distribution of the dynamic pressure of the dynamicpressure generating groove itself on the shaft-exposing side, thedynamic pressure on the non shaft-exposing side is set higher than thaton the shaft-exposing side, so that the dynamic pressure on the nonshaft-exposing side becomes higher than the dynamic pressure on theshaft-exposing side, without fail.

As a result, the shaft 100 in FIG. 5 is pulled toward the inside of theretention member 120, so that the problem of the shaft 100 lifting neverhappens. The lubricant 150 is always made to flow toward the inside aswell, and further, as surrounded by the seamless retention member 120, areliable bearing unit can be inexpensively provided in which lubricantis prevented from leaking.

In addition, the bearing unit of the present invention is used as abearing unit of a so-called fan motor, as shown in FIGS. 1 to 3. A fanmotor is a kind of a rotary drive apparatus. The bearing unit accordingto the present invention can be used needless to say as a bearing of apump apparatus or a disk drive apparatus, for example, a hard disk driveapparatus, an optical disk apparatus or a magneto optical diskapparatus, which are other examples of a rotary drive apparatus.

As described above, according to the present invention, lubricant doesnot leak, reliability can be enhanced, and also the problem of shaftdefect at the time of the rotation of a rotor caused by the imbalance ofa pair of dynamic pressure generating grooves can be solved reliably andinexpensively.

1. A bearing unit that supports a shaft in a rotatable manner,comprising: a shaft including an exposed end, an inner end having asmall external diameter provided opposite to said exposed end, and astepped middle portion having a small external diameter formed at aposition between said exposed end and said inner end; a retention memberwhich exposes said exposed end of said shaft to the outside through agap and has a seamless structure; a bearing disposed inside saidretention member, which has a first dynamic pressure generating grooveon said exposed end side and second dynamic pressure generating grooveon said inner end side formed on an inner circumferential surface facingsaid shaft and which supports said shaft in a rotatable manner in theradial direction; a thrust bearing formed inside said retention member,which supports said inner end of said shaft in a rotatable manner in thethrust direction; and lubricant in said retention member, which isfilled between said shaft, said radial bearing and said thrust bearing;wherein the length m of said inner end of said shaft in the direction ofthe shaft is shorter than the length n in the direction of the shaft ofthe part between the outer surface of said retention member and saidstepped middle portion of said shaft.
 2. A bearing unit according toclaim 1, wherein said inner end is a diminishing tapered portion or astepped portion having a small external diameter.
 3. A bearing unitaccording to claim 2, wherein the external diameter D of said inner endis larger than the external diameter d of said stepped middle portion.4. A bearing unit according to claim 3, wherein said stepped middleportion is a stepped portion formed such that the peripheral portion ofsaid shaft facing said first dynamic pressure generating groove becomessmaller on said exposed end side.
 5. A bearing unit according to claim1, wherein said first and second dynamic pressure generating grooves areherringbone grooves, and the inflow angle α of said first dynamicpressure generating groove is larger than the inflow angle β of saidsecond dynamic pressure generating groove.
 6. A rotary drive apparatushaving a bearing unit which supports a shaft in a rotatable manner,comprising: a shaft including an exposed end, an inner end of a smallexternal diameter provided opposite to said exposed end, and a steppedmiddle portion formed at a position between said exposed end and saidinner end; a retention member that exposes said exposed end of saidshaft to the outside through a gap and has a seamless structure; abearing disposed inside said retention member, which has a first dynamicpressure generating groove on said exposed end side and second dynamicpressure generating groove on said inner end side formed on an innercircumferential surface facing said shaft and which supports said shaftin a rotatable manner in the radial direction; a thrust bearing formedinside said retention member, which supports said inner end of saidshaft in a rotatable manner in the thrust direction; and lubricant insaid retention member, which is filled between said shaft, said radialbearing and said thrust bearing; wherein the length m of said inner endof said shaft in the direction of the shaft is shorter than the length nin the direction of the shaft of the part between the outer surface ofsaid retention member and said stepped middle portion of said shaft.